Compound of 脇 (waki, “armpit, side of the torso”) +‎ 差 (sashi, the 連用形 (ren'yōkei, “continuative or stem form”) of verb 差すsasu “to insert, to stick into”), from the way the wakizashi would be stuck at one's side through the 帯 (obi, “belt or sash”).[1] The sashi changes to zashi due to rendaku (連濁).